The field of the invention is magnetic resonance imaging (“MRI”) systems and methods. More particularly, the invention relates to systems and methods for deriving new clinically-useful information from a plurality of contrast mechanisms.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins,” after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude, A0, is determined by the magnitude of the transverse magnetic moment Mt. The amplitude, A, of the emitted NMR signal decays in an exponential fashion with time, t. The decay constant 1/T*2 depends on the homogeneity of the magnetic field and on T2, which is referred to as the “spin-spin relaxation” constant, or the “transverse relaxation” constant. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation signal B1 in a perfectly homogeneous field. The practical value of the T2 constant is that tissues have different T2 values and this can be exploited as a means of enhancing the contrast between such tissues.
Another important factor that contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process that is characterized by the time constant T1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T1 time constant is longer than T2, much longer in most substances of medical interest. As with the T2 constant, the difference in T1 between tissues can be exploited to provide image contrast.
Thus, images weighted based on the T1 or T2 time constants can be referred to as relaxation weighted imaging; however, a variety of other contrast mechanisms have also been developed. For example, a so-called diffusion weighted imaging (“DWI”) pulse sequence uses motion sensitizing magnetic field gradients to obtain images having contrast related to the diffusion of water or other fluid molecules. Specifically, a DWI pulse sequence applies diffusion sensitizing magnetic field gradients in selected directions during the MRI measurement cycle to obtain MR images that have an image contrast related to the diffusion of water or other fluid molecules that occurred during the application of the diffusion gradients. Using these DWI images, an apparent diffusion coefficient (“ADC”) may be calculated for each voxel location in the reconstructed images.
Though the particular information sought in given clinical application may dictate a desired contrast mechanism (for example, T1 weighting, T2 weighting, diffusion weighting, perfusion imaging, and the like), it is well known that biological tissue is often heterogeneous and, therefore, has heterogeneous MR parameters, including T1 relaxation times, T2 relaxation times, diffusion coefficients, and magnetization transfer (MT), to name but a few. Accordingly, in many clinical settings, it would often be desirable to perform multiple MR acquisitions, each focusing on different contrast mechanisms, to ensure that the clinician is provided with a broad spectrum of information that, ideally, provides a full and accurate picture of the subject.
For instance, during acute stroke, the diffusion of ischemic lesion decreases significantly. While for chronic stroke and tumor tissue, diffusion often is elevated due to edema and change in vasculature. As such, it is often desirable to perform multiple MR studies to acquire diffusion-weighted images, as well as other measurements to probe heterogeneous tissue damage. However, in many clinical settings and, particularly, when time is of the essence to diagnose and intervene to minimize the potential impact of the underlying conditions, it may be impractical to perform a large number of extended imaging studies.
Furthermore, MRI is known to be susceptible to partial volume effects, due to limited spatial resolution. Such limitations may be particularly severe when considering complex pathophysiological changes during disease states. For instance, ischemic tissue has an elevated T2 relaxation constant within hours after hypoperfusion. However, the diffusion rate of ischemic tissue has a complex pattern, whereby it initially decreases but then slowly recovers in about one week. Again, when looking to consider complex pathophysiological changes resulting during changes in disease states, it is desirable, yet not always cost effective to conduct a series of imaging studies spanning a variety of MR parameters.
Accordingly, it would be desirable to have a system and method that provides a clinician with the ability to acquire information about a variety of contrast mechanisms and MR parameters without requiring lengthy and/or repetitive imaging studies.